This invention relates to a liquid chromatograph-direct coupled mass spectrometer, which will be hereinafter referred to as LC/MS, and more particularly to a liquid inlet port in the interface of the LC/MS, which is suitable for improving the ionization efficiency without impairing the separation condition of liquid chromatography.
What is important in an LC/MS is how to introduce a sample into the mass spectrometer without impairing the separation ability in the liquid chromatography. Thus, it is desirable that an LC/MS can perform measurements without changing the measurement conditions of liquid chromatography. In an LC/MS using an ionization system based on ion-molecule reactions, however, some species of solutes in a mobile phase solvent of liquid chromatography cannot be ionized in some cases. Generally, ionization based on ion-molecule reaction under a pressure from the atmospheric pressure to a vacuum of about 1 Torr includes several types of reactions. That is, proton transfer reaction and ion clustering reaction (addition reaction) take place as principal types of reactions in the ionization.
Mobile phase solvent molecules A are ionized by corona discharge and electron bombardments and formed into reagent gas ions, i.e. AH.sup.+ ions through further collisions further with neutral molecules. Then, AH.sup.+ ions further collide with solute molecules B, whereby protons H.sup.+ are transferred from the AH.sup.+ ions to the solute molecules B to ionize the solute molecules B according to the following reaction equation. EQU AH.sup.+ +B .revreaction.A+BH.sup.+
This proton transfer reaction has a high reaction rate and is one of the most important reactions taking part in the chemical ionization. In order to make the reaction proceed toward the right side in the foregoing reaction equation, the proton affinity of mobile phase solvent molecules A must be lower than that of solute molecules B, whereby the protons H.sup.+ can be smoothly transferred from A to B. In order to efficiently ionize B, a larger difference in the proton affinity is desirable between the mobile phase solvent molecules A and the solute molecules B. Proton affinity of solvents often used in liquid chromatography is 173.7 kcal/mole with H.sub.2 O, 186.5 kcal/mole with methanol and 191.4 kcal/mole with acetonitrile. If the proton affinity of solute molecules B is more than 191.4 kcal/mole, ionization can proceed efficiently with any of these solvents, whereas in case of solute molecules B having a proton affinity of, for example, 190 kcal/mole, ionization can proceed with water or methanol as a mobile phase solvent, but not with acetonitrile. Thus, it is necessary in the chemical ionization to check the proton affinities of mobile phase solvent molecules A and the solute molecules B. In liquid chromatography, however, the mobile phase solvent plays a role of separating solutes and thus free selection of mobile phase solvents to improve the ionization efficiency is very limited. That is, generally it is not possible to simply replace acetonitrile with water as a mobile phase solvent because the ionization cannot be carried out with acetonitrile. In order to solve this problem, it has been proposed to use an optimum mobile phase solvent playing role of separating solute components in a liquid chromatography, add another solvent suitable for ionizing separated solute components and feed the resulting mixture to a nebulizing means [Anal Chem. 51 No. 14, 2324-2326 (1979); Biochemical and Environmental Mass Spectroscopy, Vol. 15, 179-182 (1988)].
FIG. 1 shows a conventional system for nebulizing an eluate in an LC/MS, where a mobile phase solvent as an eluent, stored in an eluent storage tank 1 is fed to a separation column 4 through a sample injector 3 by driving a pump 2. A sample in a solution form is injected into the eluent at the sample injector 3 and separated into solute components through the separation column 4. When the eluent is not suitable for the ionization, the eluate from the separation column 4 is led to a mixing column 5 provided downstream of the separation column 4, where the eluate is mixed with an ionization solvent fed from an ionization solvent storage tank 6 by driving a pump 2'. The mixing column 5 is filled with glass beads, and the resulting-mixture is led to a nebulizing means. However, in the proposed art, the separated solute components in the eluate from the separation column 4 are diffused and remixed in the mixing column 5, forming the so-called dead volumes. Thus, no satisfactory separation is expected. Furthermore, it is a premise of the proposed art that the mobile phase solvent and the ionization solvent are well mutually miscible and thus there is a limit to selection of an ionization solvent. For example, in the case of hexane as a mobile phase solvent, water cannot be used as an ionization solvent.
Recently, a nebulizing means for the interface region of LC/MS using a thermospray system or an atmospheric pressure ionization (API) system has been proposed to attain direct and rapid vaporization of an eluate from the liquid chromatograph and successive ionization reactions [Anal Chem, 58 No. 14 1451A-1461A (1986); Anal Chem, 55, 2280-2284 (1983)]. However, the proposed nebulizing art still has such problems as use of auxiliary gases such as nebulizer gas and makeup gas in the vaporization of eluate, and consequent unstable ionization reactions and poor separation ability in the mass spectroscopy.